1. Field of the Invention
The present invention relates to a method and device for eliminating oxygen contained in aqueous monomer solutions.
2. Description of Background Art
Oxygen inhibits and affects the polymerization of monomers and also, in particular, the free-radical polymerization of water-soluble monomers in the production of water-soluble or water-swellable polymer products used inter alia as flocculants, dehydration or retention agents, or as superabsorbers or thickening agents.
Extensive removal of oxygen prior to and during polymerization prevents inhibition or termination of the polymerization reaction and, particularly in industrial production processes, enables controlled induction and progress thereof and thus, the manufacture of the desired polymer products.
Therefore, the past has seen numerous attempts of providing processes and/or devices by means of which oxygen is removed from the monomer solutions. As a rule, these processes are based on stripping the oxygen from the monomer solution using an inert gas. Thus, for example, DE 35 40 994 teaches intimate mixing of the monomer solution with nitrogen in a two-fluid nozzle, thereby removing the oxygen from the monomer solution. However, this procedure is disadvantageous in that the nozzle tends to become blocked and/or sticky as a result of polymer formation, so that oxygen removal is liable to give trouble. Moreover, long downtimes constantly occur as a result of necessary maintenance and, in addition, the consumption of inert gas in this process is comparatively high.
It is therefore the object of the present invention to provide a process for removing oxygen from monomer solutions, which process has low susceptibility to failure and a very low specific consumption of inert gas and permits continuous operation.
According to the invention, the object is accomplished by providing a continuous process for removing oxygen from monomer solutions using an inert gas in a column-shaped apparatus.
Preferably, the apparatus is a cylindrical container, the diameter to height ratio of which being from 0.95 to 0.1, preferably from 0.8 to 0.3, and more preferably from 0.5 to 0.35.
The inert gas preferably is dispersed at the bottom of the apparatus, advantageously in the form of fine bubbles, introduced, and withdrawn at the head of the apparatus. Dispersion of the inert gas preferably is effected using a membrane or a pipe socket. Preferred inert gases are nitrogen and/or carbon dioxide. The monomer solution preferably is added at the head of the apparatus, flows through the apparatus as a liquid column, and preferably is withdrawn slightly above the bottom. The residence time of the liquid in the apparatus is varied by means of the filling level and flow rate.
In a preferred embodiment, at least one section of the liquid column is subjected to mixing. Preferably, multiple sections of the liquid column are subjected to mixing. The height of each section preferably is from 5 to 50%, more preferably from 10 to 30% of the liquid column overall height. In case of two sections; the height of the upper section preferably is from 5 to 50%, more preferably from 10 to 30%, and the height of the lower section preferably from 10 to 30% of the liquid column overall height. Preferably, said mixing extends across the entire cross-section of the liquid column. Said mixing advantageously is effected in a turbulent fashion. It is particularly advantageous to perform said mixing using stirring elements, preferably turbine agitators and/or dispersing disks. It is particularly preferred that each one section is mixed by two turbine agitators, two dispersing disks, or by one turbine agitator and one dispersing disk, the dispersing disk preferably being arranged below the turbine agitator. Thereby, the radial stirring effects, i.e., the radial flow directions are utilized by these stirring elements.
The height of the sections depends on the type of aqueous monomer solution. When producing polyelectrolytes containing cationic monomers, the height is selected to be higher. Multiple sections can be arranged in parallel or in series.
The process of the invention is advantageous in that the specific consumption of inert gas for complete removal of oxygen is exceedingly low. By varying the filling level and flow rate, it is possible to render the most diverse monomer solutions completely free of oxygen. The downtimes due to maintenance are very short in the process of the invention, and the time intervals between maintenance operations are longer.
Another object of the present invention is to provide an apparatus in which the process of the invention can be performed with particular advantage and which is easy to clean and to maintain.
According to the invention, said object is accomplished by providing an apparatus which can be filled at least in part with a liquid column and which has
a gas inlet, preferably a gas dispersing means, and a gas outlet,
a liquid inlet and a liquid outlet, and
at least one means used to mix a section of the liquid column, preferably in a turbulent fashion.
Preferably, the apparatus is cylindrical and has a diameter to height ratio of from 0.95 to 0.1, preferably from 0.8 to 0.3, and more preferably from 0.5 to 0.35.
In a preferred embodiment, the gas dispersing means is arranged at the bottom of the apparatus. Gas dispersion preferably is performed through a membrane or a pipe socket.
Preferably, the means for mixing is a stirring element. In a preferred embodiment, the stirring element is a turbine agitator and/or a dispersing disk. In a particularly preferred fashion, the stirring element consists of two turbine agitators, two dispersing disks, or one turbine agitator and one dispersing disk, the turbine agitator preferably being arranged above the dispersing disk. The stirring element is arranged on a shaft driven by a motor which preferably is situated at the head of the apparatus. Using said stirring element, the liquid column is subjected to locally limited mixing, preferably in a turbulent fashion. Advantageously, 5-50%, preferably 10-30% of the overall height of the liquid column and preferably the entire cross-section thereof is mixed by each stirring element. The apparatus according to the invention preferably has multiple stirring elements, more preferably two stirring elements; preferably 5-50%, more preferably 10-30% of the overall height of the liquid column is mixed by the upper one, and preferably 10-30% by the lower one.
In a preferred embodiment, the apparatus according to the invention additionally has internal members preferably designed in a disk shape and extending across part of the cross-section of the apparatus so as to avoid back-mixing of the liquid column as much as possible. These internal members preferably are of annular shape and preferably arranged above the stirring elements.
The apparatus preferably has an automatic filling level control, by means of which the height of the liquid column is maintained at a specific level.
The apparatus according to the invention preferably is used to remove oxygen from a monomer solution. In this event, an oxygen measuring means preferably is arranged in the outlet of the apparatus, which is used to monitor the oxygen content and optionally can be employed in the automatic control of oxygen removal. If the measured oxygen content is too high, the filling level of the container is increased and/or the flow rate is reduced. In the event of an excessively high oxygen concentration, it is also possible to increase the flow of the inert gas volume.
The apparatus of the invention may also be used in flotation and in any type of mass transfer and/or reaction process.
The apparatus according to the invention is readily produced, easy to maintain, and has little susceptibility to failure. When used according to the process of the invention, the service time, i.e., the time interval between two maintenance operations is exceedingly long compared to the use of other apparatus for such purposes. At the same time, the specific consumption of nitrogen is lower compared to apparatus according to prior art. Multiple apparatus can be arranged in parallel or in series. In a highly flexible fashion, the apparatus can also be employed for most various purposes and provided with specific gas dispersing means and/or internal members to this end. The apparatus according to the invention can be used in continuous or batch processes. In case of production intermissions, for example, the monomer solution in the apparatus can be restabilized by gassing with air and put to safe storage. A short time before restarting, the monomer solution again is made free of oxygen using inert gas.
With reference to two examples, the apparatus of the invention will be illustrated hereinbelow. The explications are merely by way of example and are thus not limiting to the general idea of the invention.